Reportable Segments - NOI for Each Segment from Our Rental Real Estate Specific to Continuing Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Rental income	$ 639,620	$ 612,299	$ 1,272,451	$ 1,216,219
Operating income	217,654	210,550	414,523	414,921
Total Assets	20,406,582		20,406,582		$ 20,570,599
Capital Expenditures			85,987
Same store
Segment Reporting Information [Line Items]
Rental income	599,628	586,757	1,183,348	1,158,193
Operating expenses	177,679	172,104	355,724	343,753
Operating income	421,949	414,653	827,624	814,440
Total Assets	17,456,473		17,456,473
Capital Expenditures			83,659
Non-same store/other
Segment Reporting Information [Line Items]
Rental income	39,992	25,542	89,103	58,026
Operating expenses	15,547	14,715	37,618	27,402
Operating income	24,445	10,827	51,485	30,624
Total Assets	2,950,109		2,950,109
Capital Expenditures			2,328
Same Store, Non-same Store and Other
Segment Reporting Information [Line Items]
Rental income	639,620	612,299	1,272,451	1,216,219
Operating expenses	193,226	186,819	393,342	371,155
Operating income	446,394	425,480	879,109	845,064
Other markets | Same store
Segment Reporting Information [Line Items]
Rental income	481	461	969	924
Operating expenses	175	144	341	340
Operating income	306	317	628	584
Total Assets	12,795		12,795
Capital Expenditures			102
Los Angeles | Same store
Segment Reporting Information [Line Items]
Rental income	102,619	99,921	205,103	198,557
Operating expenses	29,032	28,309	58,892	56,960
Operating income	73,587	71,612	146,211	141,597
Total Assets	2,649,731		2,649,731
Capital Expenditures			13,220
Orange County | Same store
Segment Reporting Information [Line Items]
Rental income	22,829	21,998	45,331	43,654
Operating expenses	5,564	5,376	11,090	10,838
Operating income	17,265	16,622	34,241	32,816
Total Assets	323,604		323,604
Capital Expenditures			4,017
San Diego | Same store
Segment Reporting Information [Line Items]
Rental income	22,932	21,897	45,425	43,620
Operating expenses	5,788	5,736	11,751	11,535
Operating income	17,144	16,161	33,674	32,085
Total Assets	413,701		413,701
Capital Expenditures			2,481
Subtotal - Southern California | Same store
Segment Reporting Information [Line Items]
Rental income	148,380	143,816	295,859	285,831
Operating expenses	40,384	39,421	81,733	79,333
Operating income	107,996	104,395	214,126	206,498
Total Assets	3,387,036		3,387,036
Capital Expenditures			19,718
San Francisco | Same store
Segment Reporting Information [Line Items]
Rental income	115,332	111,835	219,400	213,516
Operating expenses	27,581	28,466	52,854	54,018
Operating income	87,751	83,369	166,546	159,498
Total Assets	3,004,183		3,004,183
Capital Expenditures			21,259
Washington DC | Same store
Segment Reporting Information [Line Items]
Rental income	108,689	107,103	215,843	213,546
Operating expenses	33,237	32,241	66,899	64,572
Operating income	75,452	74,862	148,944	148,974
Total Assets	3,742,083		3,742,083
Capital Expenditures			14,137
New York | Same store
Segment Reporting Information [Line Items]
Rental income	120,498	120,005	240,124	239,367
Operating expenses	46,680	43,499	94,505	88,938
Operating income	73,818	76,506	145,619	150,429
Total Assets	4,404,182		4,404,182
Capital Expenditures			12,092
Boston | Same store
Segment Reporting Information [Line Items]
Rental income	57,217	55,943	113,511	110,989
Operating expenses	15,511	14,941	31,593	30,145
Operating income	41,706	41,002	81,918	80,844
Total Assets	1,609,747		1,609,747
Capital Expenditures			9,451
Seattle | Same store
Segment Reporting Information [Line Items]
Rental income	49,031	47,594	97,642	94,020
Operating expenses	14,111	13,392	27,799	26,407
Operating income	34,920	34,202	69,843	67,613
Total Assets	1,296,447		1,296,447
Capital Expenditures			6,900
Non-same store | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	39,427	13,850	83,193	34,439
Operating expenses	13,154	5,193	29,862	12,533
Operating income	26,273	8,657	53,331	21,906
Total Assets	2,414,779		2,414,779
Capital Expenditures			2,210
Other [Member] | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	565	11,692	5,910	23,587
Operating expenses	2,393	9,522	7,756	14,869
Operating income	(1,828)	$ 2,170	(1,846)	$ 8,718
Total Assets	$ 535,330		535,330
Capital Expenditures			$ 118